This application relates to an apparatus and method for optical inspection of semiconductor wafers. In particular, the apparatus and method of the present invention provide high-throughput inspection of wafers using a plurality of independent good sensitivity, low-cost inspection devices whose packaging and form factor allow operation in parallel. The present invention may be applied to brightfield and/or darkfield inspection systems.
Optical inspection of semiconductor wafers has become a standard step in the production of semiconductors. Wafers are illuminated with light emanating from a controlled illuminator, and an image of the surface is constructed based on the portion of the light reflected or otherwise directed to a light sensor. The image is processed to isolate defects from valid structures.
The sensitivity of optical inspection systems to small sized defects on a patterned wafer surface is determined by the ability of the system to discriminate between a defect signal and a valid structure signal. In brightfield inspection systems, sensors register light reflected from the wafer surface typically achieving pixel-to-defect ratio sensitivities of 1:1 or 2:1. In darkfield systems, sensors register the scattered and diffracted light, the light which deviates from a perfect reflection. In contrast to brightfield systems, darkfield systems can easily achieve a 10:1 or 20:1 pixel-to-defect ratio for certain common defects.
Optical inspection systems are either imaging or non-imaging. In imaging systems, a lens captures light reflected from an area on the wafer surface and preserves the spatial information encoded in that light (e.g., a spatial distribution of light intensity). Sensors are typically arrays of light-sensitive detectors such as charge-coupled device (CCD) xe2x80x9ccamerasxe2x80x9d or, more recently, CMOS photodiode or photogate cameras.
In contrast, in non-imaging systems the light from the illuminator is concentrated on a small area (ideally a very small point) on the wafer""s surface. A sensorxe2x80x94for example a photomultiplier tube, photodiode, or avalanche photodiodexe2x80x94detects scattered, or diffracted light, and produces a signal proportional to the integrated light intensity.
Others have concentrated on attempting to maximize sensitivity in order to detect the defects of the smallest size. Consequently, expensive special-purpose optical, mechanical, electronic, and computer processing systems have been employed in state-of-the-art tools. For example, these systems may allow adjustments in magnification levels, illumination angles, and polarization, each of which further increases system complexity and cost. The components in each sensing subsystem of the present invention are limited to performing only one type of inspection, further reducing cost by eliminating complexity. The present invention""s individual subsystems emphasize compactness and low-cost over sensitivity, providing a system with medium sensitivity and high levels of throughput. Increasing the throughput of such systems stresses the design of these components and further increases the cost.
Some applications do not require the high level of sensitivity produced by such systems. In equipment monitoring, for example, a statistical process may be used to indicate whether the equipment used during a manufacturing step is functioning correctly. The sensitivity, in this case, need only be high enough to detect when the manufacturing equipment is causing a statistically significant excess number of defects (excursion). Since this type of application is most useful if repeated often (after every manufacturing step, for example), fast examination speed is important to keep up with the manufacturing process.
What is needed therefore is a fast and relatively inexpensive tool for monitoring the condition of wafers at various points in the integrated circuit manufacturing process.
The present invention addresses the problem by providing a high-throughput inspection system at low cost, particularly for equipment monitoring applications. A single low-cost unit can be provided using xe2x80x9cconsumer gradexe2x80x9d devices for illumination, imaging, image sensing, and image-processing which can measure a part of the wafer by itself with acceptable sensitivity, and with very high speed. A system can be provided utilizing a plurality of single units which can be scanned over the wafer together to complete the measurement. Miniaturization provides a form factor such that a plurality of the units can be packaged together. Each unit operates independently scanning a swath across the wafer parallel to adjacent units. Together, the stacked parallel units scan the wafer. The cost savings using the consumer grade components, even with a plurality of units, is large compared to the system of others while providing comparable sensitivity and faster throughput.
The components in each sensing subsystem of the present invention generally perform one type of inspection (e.g., imaging scattered radiation from a semiconductor substrate), further reducing cost by eliminating complexity. The present invention""s individual subsystems emphasize compactness and low-cost over versatility, providing a system with good sensitivity and high levels of throughput.
In one aspect, the invention provides a multi-stage integrated circuit manufacturing system employing a modular optical inspection system for inspecting a semiconductor wafer. The manufacturing system may be characterized as including the following features: (a) a plurality of interrelated integrated circuit manufacturing tools capable of operating in parallel on a plurality of semiconductor wafers; (b) a modular optical inspection system; and (c) a handling tool for moving the semiconductor wafers among the plurality of manufacturing tools and the inspection system. The modular optical inspection system may include (i) a plurality of modular inspection subsystems each configured to detect defects on a portion of a semiconductor wafer, and (ii) a mechanism for moving at least one of the semiconductor wafer and the plurality of modular inspection subsystems with respect to one another. Each of the modular inspection subsystems has a field of view spanning a fraction of the width of the semiconductor wafer. Together the subsystems"" fields of view cover a substantial portion of the wafer surface.
In operation, the manufacturing system transfers the semiconductor wafer (at some arbitrary point in the fabrication process) from one of the plurality of manufacturing tools to the modular optical inspection system. Then, the inspection subsystem moves at least one of the semiconductor wafer and the plurality of modular inspection subsystems with respect to one another such that each of the modular inspection subsystems inspects, in a single pass across the semiconductor wafer, an associated region of the semiconductor wafer.
The interrelated integrated circuit manufacturing system may be a cluster tool or a phototrack tool for example. In a preferred embodiment, the multi-stage integrated circuit manufacturing system includes a cooling station at which the modular optical inspection system is located. To avoid difficulties associated with introducing the inspection system into a vacuum environment associated with cluster tools and other integrated circuit manufacturing systems, the modular optical inspection system may be located above a window of one of the manufacturing tools (e.g., a cooling stage). Then the system provides the wafer at a location near the window so that the inspection system can take an image.
Another aspect of the invention provides a modular optical inspection subsystem as described abovexe2x80x94although not necessarily used with a multi-stage manufacturing systemxe2x80x94but including a translatable Fourier filter system. The Fourier filter system may include the following features: (a) a translatable medium having transparent regions and opaque regions in fixed spatial relation to one another and defining multiple Fourier filters; and (b) a translation mechanism arranged to translate the translatable medium such that individual Fourier filters are presented for filtering light. In one embodiment, the translatable medium is a tape that may be wound on rollers (or reels) or a continuous loop.
The translatable medium may assume various configurations. In one case, the spacing between the opaque regions varies substantially continuously over at least a segment of the translatable medium defining at least two Fourier filters. Alternatively, at least a segment of the translatable medium is divided into discrete Fourier filters. The translatable medium may be made from etched metal, metallized transparent plastic, etc. In some embodiments, parallel strips define the opaque regions. This is most appropriate for an etched metal design. In other embodimentsxe2x80x94appropriate for the metallized plastic constructionxe2x80x94the opaque regions may comprise discrete spots.
The translation mechanism may include any components suitable for translating the medium to present differing Fourier filters to a Fourier image plane in a modular inspection subsystem. For example, the mechanism may include an actuator (preferably only one), a mechanism arranged to engage an actuator, a roller which rotates under torque from the actuator, etc.
In another aspect of the invention, a modular optical inspection system as described above may employ time-delay integration to facilitate rapid imaging. In such systems, at least one of the plurality of modular inspection subsystems includes (a) a two-dimensional sensor configured to receive light from the surface of the semiconductor wafer, and (b) a controller configured to control the relative speeds at which (i) data is read from the sensor and (ii) the modular inspection subsystem and the semiconductor wafer are moved with respect to one another. In operation, the controller causes one row of pixel data to be read from the two-dimensional sensor each time the at least one inspection subsystem moves by one pixel length with respect to the semiconductor wafer. The two-dimensional sensor may be a CCD array, for example.
Yet another aspect of the invention provides a particular data processing system for the modular optical inspection system as described above. In this case, the inspection system may include (a) a master processor configured to process data delivered from at least some of the modular inspection subsystems and (b) a local processor provided with a first one of the plurality of modular inspection subsystems and configured to process data collected by the first modular inspection subsystem. Typically, the master processor is connected to and directly communicates with each of the separate local processors. In a preferred embodiment, the local processor is a digital signal processor. Regardless of the class of processor, the local processors may implement an algorithm that distinguishes valid pattern scattering from defect scattering on the semiconductor wafer.
The features and advantages of this invention may be further appreciated with reference to the following detailed description and associated drawings.